The discovery of high temperature superconductors has lead to the development of a number of applications for their use. Superconductors are known to have the property that they have zero direct current (DC) resistance below a critical temperature Tc. They also have zero DC resistance below a critical current Ic and a critical magnetic field Bc.
One potential use of HTS is in FCLs. HTS can be used in FCLs in a number of ways, and the use of HTS to limit fault currents is an elegant solution to the ever-present short circuit threat in power networks.
There are several different methods to incorporate HTS in an FCL design. FIG. 1 illustrates schematically a sectional view through a known form of DC saturated HTS FCL. Arrangement 1 of FIG. 1 utilises two separate closed iron cores 2, 3. Each of the cores has a separate DC HTS coil winding, with a first winding including sectional portions 5, 6 and a second winding including sectional portions 7, 8. Each of the DC HTS coil windings contains N turns.
Similarly, two series of alternating current (AC) linkage windings, including a first winding having sectional portions 10, 11 and a second winding having portions 12, 13 are also provided, with each of the windings having n turns.
Each of the iron cores structures has a given height h and a given width w. During operation, each core is saturated to a predetermined flux density value φdc of opposite sense, with the opposite sense being indicated by standard dot notation 16, 17. The DC current flows out of the page 16 in the positive cycle saturated core 5, 6, and into the page 17 in the negative cycle saturated core 7, 8. These points on the DC magnetisation curve of the cores are represented as ±Bdc and ±Hdc, respectively.
The required ampere-turns of each HTS DC coil 5, 6 and 7, 8 is given byNI=2(2w+2h)Hdc  (1)where N is the number of DC turns, I is the HTS coil excitation DC current, w is the effective core structure width in the plane of the paper of FIG. 1, h is the effective core structure height in the plane of the paper of FIG. 1, and Hdc is the design value for the saturation of the core.
The AC windings 10, 11 and 12, 13 are then arranged such that the differential permeability μdiff from each AC coil is in the opposite sense to each windings' core magnetisation. The variable, μdiff is defined byμdiff=(dB/dH)|average=ΔB/ΔH  (2)where ΔB and ΔH are the maximum extents of the minor hysteresis loop at the DC bias points ±Bdc and ±Hdc, respectively.
In addition, the relative differential permeability may be defined asμdiff=μdiff/4π*10−7.For reference, the magnetic reluctance of the iron core presented to the DC coil isR=(Hl)/(BA)=l/μA  (3)where R is the magnetic reluctance [H−1], B is the magnetic field [T], A is the cross-sectional area of the iron core (not including any insulation or varnished area) [m2], μ is the magnetic permeability of the iron core [Hm−1], l is the magnetic length of each core that is approximately equal to 2 w+2 h [m], and H is the magnetic induction (NI/l) [Am−1].
The steady state AC impedance presented to the network line in which the core is in series may be expressed in phasor notation asZ=R+2πf(n2A/l)μdiffi  (4)where R is the resistance of the AC coils, f is the frequency of operation (i.e. 50 Hz), i is the square root of −1 (the imaginary number), and n is the number of turns of the AC winding. R is normally negligible compared to the imaginary part of the impedance. For an effective HTS FCL, the normal operating inductance of the core must be small so as not to impose any unnecessary regulation of the line or impedance to the current flow. This is normally achieved by ensuring that Bdc is greater than 1.5 T, and thereby ensuring that μdiff is approximately 1, the device thereby behaving effectively as an air core inductor.
In operation, the DC field is chosen such that an oscillatory fault current of peak value If, determined by the network impedance and surge characteristics, increases the differential permeability to that of the maximum DC value. The size of the cores, DC current, and DC turns can be calculated based on the fault level and the permeability of the iron so thatnIf(max)/l=Hdc  (5)andnIf(min)/l=Hdc−Hdc(sat)  (6)where n is the number of AC turns, l is the length of the magnetic circuit, Hdc is the DC field intensity at which the iron core has a maximum μdiff, Hdc(sat) is the field intensity required to saturate the core, If(max) is the maximum fault current that the HTS FCL is required to limit and If(min) is the minimum fault current that the HTS FCL is required to limit.
Owing to the oscillatory nature of a fault current, two separate cores 20, 21, as shown in FIG. 1, are normally required to provide different senses of the AC coil current to the AC windings, as fault currents are oscillatory in nature, and require limiting on both the positive and negative parts of each cycle. Using this concept, a three-phase HTS FCL would require six saturated cores, which would entail six separate HTS DC windings with associated cryogenics. Such a large number of HTS DC windings adds significantly to the expense of the overall device.